Chemical contamination of subsurface environments damages local ecosystems and poses health risks where groundwater is used as a source of drinking or irrigation water. Such contamination emanates from various industrial and municipal sources including chemical storage sites, landfills, transportation facilities, and storage tanks located above ground and underground.
A number of methods for treating contaminated soil and groundwater have been available for some time. For example, soil may be excavated, treated at an off-site facility, incinerated and/or disposed. Other methods involve bioremediation techniques. Bioremediation methods employ natural processes to degrade contaminated soil or water. Such methods effectively treat a variety of contaminants. For example, contaminated groundwater may be pumped to the surface and treated to remove or degrade contaminants; similarly, contaminated soil can be removed from a site and treated with biological organisms (Buchanan, U.S. Pat. No. 5,622,864; Stoner et al., U.S. Pat. No. 5,453,375). These methods, however, tend to be expensive and laborious, they require long times for effective treatment, and they carry the risk of exposing contaminants to the atmosphere.
Alternative bioremediation techniques known in the art provide a supply of nutrients in situ via injection wells, thereby circumventing the need to pump or otherwise move contaminated material to the ground surface. These techniques increase bioremediation rates by furnishing heightened concentrations of nutrients to indigenous microbial populations that are capable of degrading contaminants. For example, Looney et al. (U.S. Pat. No. 5,480,549) describe a method by which vapor-phase phosphates such as triethylphosphate and tributylphosphate are metered into a gas stream that is injected via injection wells into contaminated soil and groundwater to stimulate the microbial degradation of hydrocarbon contaminants. The effectiveness of this method, however, is limited to the biodegradation of hydrocarbon contaminants, and is thus inadequate to bioremediate sites where more pernicious contaminants such as halogenated hydrocarbons (halocarbons) persist.
Halocarbons are ubiquitous and are used for a variety of purposes such as dry cleaning agents, degreasers, solvents, and pesticides. Unfortunately, they are one of the most pervasive and harmful classes of contaminants in ground water and soil. Chlorinated hydrocarbons (chlorocarbons) such as tetrachloroethylene (PCE), trichloroethylene (TCE), dichloroethylene (DCE), and vinyl chloride (VC), are exemplary common contaminants. This class of compounds is more resistant to microbial degradation, and thus tends to persist for long periods in the environment. Because the halogen atoms in halocarbons increase the oxidation potential of the carbon atoms to which they are bound, aerobic biodegradation processes are energetically less favorable, particularly for highly halogenated compounds. Consequently, highly halogenated compounds are much more susceptible to anaerobic degradation.
Organic compounds generally act as electron donors. Polyhalogenated compounds, however, behave as electron acceptors in reducing environments as a consequence of the presence of electronegative halogen substituents. Thus, more highly halogenated compounds are less susceptible to aerobic degradation, and more susceptible to anaerobic degradation.
In the environment, halogenated compounds may be naturally dehalogenated by a variety of chemical reactions and microbe-mediated reactions. Some compounds are transformed into products which are more degradable than the parent compounds, or may be more degradable under different environmental conditions. For example, PCE which has been recently released into soil and groundwater will not have degraded much; thus degradation (dehalogenation) will operate on mostly PCE and will be most efficacious in an anaerobic environment. A very old release of PCE, however, will have been naturally dehalogenated to some extent into daughter compounds TCE, DCE, and VC, which are most readily degraded in aerobic environments.
Some environments are inhabited by chemoheterotrophic microorganisms, which may be capable of anaerobically metabolizing existing carbon sources, resulting in the evolution of excess hydrogen (H2). In the resultant reducing environment, PCE may undergo dehalogenation to TCE. Similarly, TCE may be dehalogenated to DCE and VC. As mentioned above, these latter products are not readily degraded in anaerobic conditions, but can be oxidized under aerobic conditions.
The bioremediation of soil and groundwater contaminated with highly chlorinated hydrocarbons is known in the art. Methods of stimulating the activity of indigenous microbes capable of degrading halocarbons has been achieved by treating subsurface environments with certain carbon nutrients, such as corn syrup and yeast extract (Keasling et al., U.S. Pat. No. 6,150,157) and molasses (Suthersan, U.S. Pat. Nos. 6,143,177 and 6,322,700). These methods, however, require the use of many injection wells, and are limited to the remediation of groundwater where the carbon sources are able to be dispersed. Consequently, they are not practical for the remediation of vadose zones, where the mobility of nutrients such as corn syrup and molasses is negligible. One attempt to overcome these limitations was disclosed by Hughes et al., (U.S. Pat. No. 5,602,296), whose method entails the injection of pure hydrogen (H2) into contaminated subsurface regions. Reductive dechlorination of chlorinated hydrocarbons was suggested to be mediated by indigenous anaerobic bacteria. This method, however, creates a strongly reducing environment and is thus ineffective for the degradation of partially chlorinated hydrocarbons such as DCE and VC. Moreover, it is ineffective in the treatment of nonhalogenated contaminants. Finally, hydrogen is extremely flammable, and thus poses a serious health risk where it is used as a pure gas.
Thus, there is a need in the art for a method of in situ biodegradation that is useful against a wide variety of contaminants, including halocarbons and non-halogenated compounds. The present invention satisfies these needs by overcoming the limitations of the prior art discussed above.